It has been a problem to provide a device that is able to transmit, in a secure manner, a set of high-quality visual still images in a JPEG-type format directly to a display device and/or to be recorded on the hard disk of a unit connecting the teletransmission network to the display device, while at the same time preserving visual quality, but avoiding any fraudulent utilization such as the possibility of making illegal copies of still images recorded on the hard disk of the set top box.
Using currently available solutions, it is possible to transmit still images in digital format using broadcast networks of the Hertzian, cable, satellite or other types, or through DSL (Digital Subscriber Line) or BLR (local radio loop) type telecommunication networks. Moreover, to avoid hacking of copyrighted work distributed in this way, the latter are often encrypted using various known methods.
EP 0 920 209 discloses a video scrambling process in the MPEG-2 format, where the video data consists of at least one block of at least one image encoded in the Intra (I) mode and consisting of data (AC, DC) that has been selected using a discreet cosine transform (DCT) and which, for each block, consists of a primary DC coefficient that features the average intensity of the pixels in the block and AC coefficients representing variations in intensity between pixels in the block. Video scrambling is conducted using an internal permutation step in at least one block, according to a permutation row (R), and where coefficients AC represent the variations in intensity between the pixels in the block. That requirement can be applied to JPEG-type images whose structure is similar to that of images in Intra-mode of an MPEG-2 data stream. The AC coefficients and the R-row permutation function applied to them also involve the DC coefficients after they have been recalculated using a C command. That system is a standard encryption solution and allows only scrambling and unscrambling of an MPEG-2 data stream without a security procedure for the broadcast of its data for reconstruction of the original data stream. In particular, all the data used to reconstruct the data stream (row R and DC coefficients encrypted using a key), as well as all the data constituting the original data stream, are comprised in the MPEG-2 data stream and are sent together and through the same route, where the protected MPEG-2 data stream containing the data for the reconstruction is the sole output data for the scrambling process. Consequently, EP '209 does not meet the high-security criteria.
EP 1 011 269 discloses a system for processing a data signal that consists of a system for scrambling the data signal and at least one unscrambling system for the scrambled data signal. The scrambling system comprises means for analyzing the entropic distribution of the data signal, means for scrambling a data signal as a function of its entropic distribution to form a scrambled data signal that has the same entropic distribution as the clear data signal, and means for compressing the scrambled data signal. The unscrambling system consists of means for decompressing the scrambled signal and means for unscrambling the scrambled signal to form the clear data signal. EP '269 also discloses a secure distribution system of a signal in which unscrambling the data stream requires the scrambled data stream and an additional piece of data to unscramble the data stream. Nevertheless, the additional information is stored and encrypted on the hard disk belonging to the user who wishes to utilize the signal. An ill-intentioned user who has an appropriate software means can decrypt the additional information and use the clear data signal without the consent of the owner of this signal. That system is founded solely on the entropic distribution of the signal to scramble it, and scrambling is carried out before compression and quantification. EP '269 describes how to limit the deterioration involved in the process with an equalization applied to minimize losses. However, this process is not without losses and does not guarantee the absence of deterioration. Finally, the process represents a standard encrypting and scrambling solution, where all the initial information is present in the protected data stream. Consequently, the goal of preserving the high visual quality of a process without losses and ensuring high security of the visual contents and the data allowing their reconstruction is not achieved.